Technical Field
The present disclosure relates to a toner for developing electrostatic charge image in electrophotography, electrostatic recording, electrostatic printing, etc., and a developing device and a process cartridge using the toner.
Description of the Related Art
Electrophotography is still under research and development through various inventive and technical approaches. In electrophotography, an image is generally formed by charging and irradiating the surface of photoconductor to form an electrostatic latent image, developing the electrostatic latent image into a toner image with colored toner, transferring the toner image onto a transfer medium such as paper, and fixing the toner image on the transfer medium by a heat roller, etc.
As methods for fixing toner, contact heat fixing methods, such as heat roller fixing method, are widely employed. Fixing devices for use in heat roller fixing method are generally equipped with a heat roller and a pressure roller. A recording sheet having a toner image thereon is allowed to pass the pressure-contact point of the heat roller with the pressure roller (i.e., the nip portion) so that the toner image can be melted and fixed on the recording sheet.
Resins for use in toner are generally selected from vinyl polymeric resins and polyester-backbone resins. These resins have both advantages and disadvantages in terms of flowability, mobility, chargeability, fixability, and image property. Nowadays, combined resins in which both kinds of these resins are combined and hybrid resins having both of these backbones are being used. As methods for producing toner other than conventional knead-pulverization method, the following methods are known: suspension method and emulsification method, both using an organic solvent and an aqueous solvent; suspension polymerization method that directly produces toner particles by controlling polymerizable monomer droplets; and aggregation method that produces toner particles by aggregating emulsified fine particles. These methods are called wet granulation methods or chemical toner methods.
The contact heat fixing methods can achieve energy saving as the heating temperature is lowered as much as possible. Accordingly, resins which are meltable at low temperatures are suitable for toner for use in the contact heat fixing methods. On the other hand, electrophotography has a process in which toner is mechanically or thermally stressed. Thus, the resins should be limited in thermal properties, such as glass transition temperature, so as not to cause toner blocking. The resins should also be limited in molecular weight so as not to cause toner cracking. Although such limited thermal properties and molecular weight generally do not go together, toner is required to achieve a good balance therebetween. In view of this, core-shell toners have been proposed in which an inner resin having an advantageous property for fixing is covered with an outer resin having an advantageous property for avoiding toner blocking.
In addition, core-shell toners which use polyester resins are also known. Polyester resins are generally advantageous in terms of toughness, heat resistance, and fixability.
In order not to expose release agent at the surface of toner and to maximize the exuding efficiency of the release agent when the toner is being fixed, it is preferable that the release agent have a spherical shape. So long as the release agent is contained within the toner, it is also preferable that the release agent has a larger particle diameter.